The fluid flow and heat transfer processes inside three thermal storage tanks with different top or obstacle structures are numerically analyzed in this paper to take advantage of solar energy and improve the comprehensive efficiency of solar energy system with the engineering background of Urumqi region"s solar water heating system for floor heating. The alternate operation mode of each water tank is also discussed. The results show that it is feasible to use the multi-tank alternating operation mode at different times. In actual engineering, on the premise of meeting the needs of hot users, the low-flow operation mode should be preferentially adopted. According to the thermal stratification characteristics of the water tank, 3# water tank is preferentially operated at 11:30, followed by 2# and 1# water tanks. 3# water tank should be preferentially operated at 15:00 and 18:00 while 1# and 2# water tanks can be used as backup for each other. The 3# water tank is more suitable for users with higher water temperature requirements but less water consumption, followed by the 1# water tank. When users generate a lot of backwater and need hot water with higher temperature, the 2# water tank is preferentially enabled.